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Ukrainian soldiers in a camp near Gukovo in Russia on Monday

Investigators in Russia have announced the arrest of five Ukrainian army officers for alleged war crimes as fighting rages in eastern Ukraine.

The officers from Ukraine's 72nd Mechanised Brigade are apparently among hundreds of soldiers who crossed into Russia to flee from pro-Russian rebels.

It is unclear on what grounds Russia can prosecute the officers, who reportedly deny the charges.

Meanwhile, government forces in eastern Ukraine lost 15 soldiers within a day.

Fifteen were killed and 79 injured on Thursday, the Ukrainian government said, two days after 18 were killed - the highest daily death toll reported in weeks.

Government forces have been accused of bombarding residential areas from the ground and the air, killing civilians, as they try to recapture rebel strongholds. International monitors have been surveying damage to homes.

In other developments

Ukrainian Prime Minister Arseniy Yatseniuk proposed imposing Ukrainian sanctions on Russian individuals and companies who sponsored "terrorism" in his country

A Russian war photographer, Andrei Stenin, was reported missing in east Ukraine

The remains of another 21 victims of the Malaysia Airlines plane which crashed in eastern Ukraine have now been formally identified, the Dutch government said, bringing the total to 23

An investigation by the Financial Times found that dozens of computers in the Ukrainian prime minister's office and at least 10 of Ukraine's embassies abroad had been infected with a virulent cyber espionage weapon "linked to Russia"

Arrests in Russia

In a statement (in Russian) on its website, Russia's powerful Investigative Committee (SK) announced that the five officers had been detained following questioning of 400 soldiers from the 72nd Brigade and Ukrainian border guards, who crossed into Russia on Sunday (earlier reports said Monday).

The Ukrainian service personnel were housed by Russian border guards in a tent camp near Gukovo, in Russia's Rostov region, after surrendering their weapons. Many have since returned to Ukraine under Russian escort.

Battalion commander Ivan Voitenko and four of his subordinates - Vitaliy Dubyniak, Olexander Poliakov, Olexander Ohrimenko and Dmytro Ustylko- were detained as part of an investigation into the "use of banned means and methods of conducting war", the SK said.

They are accused of using heavy weapons to bombard two towns in Luhansk region, Krasnopartizansk and Krasnodon, between 19 July and 3 August, killing "at least 10 civilians" and destroying homes.

The men admit taking part in military operations but deny attacking civilians, the Russian investigators said. They are also being investigated for attacks on Russian territory.

The Russian statement, signed by SK spokesman Vladimir Markin, ends with a political attack on the detainees, suggesting that they had been "intoxicated by nationalism".

Last month, Russian prosecutors charged a captured Ukrainian army pilot, Nadiya Savchenko, with complicity in the murder of two Russian journalists.

Border battle

Monitors from the Organization for Security and Cooperation in Europe (OSCE) have been documenting damage to civilian installations in east Ukraine.

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Ukrainian army Grad rockets in the air over Donetsk region on Thursday

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Smoke billows from the wreckage of a Mig-29 jet near Donetsk city on Thursday

On Thursday, a team visited a city hospital in Donetsk hit by shells, the OSCE said in a tweet. They also inspected damage to housing in the war-torn town of Shakhtarsk, close to the site of the flight MH17 crash last month, another tweet said.

International investigators suspended their work at the site on Thursday because of nearby fighting. The destruction of the jet has been blamed in the West on a rebel-fired missile, a charge the separatists deny.

Around 1,500 people, both civilians and combatants, have been killed since Ukraine's new government sent forces into the east in April to put down an armed uprising by the separatists.

Western sanctions against Russia over its role in events in Ukraine prompted a sharp response from the Kremlin this week when it retaliated by banning most food imports from the US and EU.